Previous: srfi shared structures rationale, Up: srfi shared structures [Index]
This SRFI creates an alternative external representation for data
written and read under write/ss and read/ss. It is
identical to the grammar for external representation for data written
and read under write and read given in section 7 of
R5RS, except that the single production:
<datum> --> <simple datum> | <compound datum>
Is replaced by the following five productions.
<datum> --> <defining datum> | <nondefining datum> | <defined datum> <defining datum> --> #<indexnum>=<nondefining datum> <defined datum> --> #<indexnum># <nondefining datum> --> <simple datum> | <compound datum> <indexnum> --> <digit 10>+
Writes a written representation of obj to the given port.
Strings that appear in the written representation are enclosed in
doublequotes, and within those strings backslash and doublequote
characters are escaped by backslashes. Character objects are written
using the #\ notation.
Objects which denote locations rather than values (cons cells, vectors,
and non–zero–length strings in R5RS scheme; also mutable objects,
records, or containers if provided by the implementation), if they
appear at more than one point in the data being written, must be
preceded by #N= the first time they are written and replaced by
#N# all subsequent times they are written, where N is a
natural number used to identify that particular object.
If objects which denote locations occur only once in the structure, then
write-with-shared-structure must produce the same external
representation for those objects as write.
write-with-shared-structure must terminate in finite time when
writing finite data. write-with-shared-structure must produce a
finite representation when writing finite data.
write-with-shared-structure returns an unspecified value.
The port argument may be omitted, in which case it defaults to the
value returned by current-output-port.
The optarg argument may also be omitted. If present, its effects
on the output and return value are unspecified but
write-with-shared-structure must still write a representation
that can be read by read-with-shared-structure. Some
implementations may wish to use optarg to specify formatting
conventions, numeric radixes, or return values. The reference
implementation ignores optarg.
For example, the code:
(begin (define a (cons 'val1 'val2))
(set-cdr! a a)
(write-with-shared-structure a))
should produce the output #1=(val1 . #1#). This shows a cons
cell whose cdr contains itself.
read-with-shared-structure converts the external representations
of Scheme objects produced by write-with-shared-structure into
scheme objects. That is, it is a parser for the nonterminal
<datum> in the augmented external representation grammar defined
above.
read-with-shared-structure returns the next object parsable from
the given input port, updating port to point to the first character past
the end of the external representation of the object.
If an end–of–file is encountered in the input before any characters
are found that can begin an object, then an end–of–file object is
returned. The port remains open, and further attempts to read it (by
read-with-shared-structure or read) will also return an
end–of–file object. If an end–of–file is encountered after the
beginning of an object’s external representation, but the external
representation is incomplete and therefore not parsable, an error is
signalled.
The port argument may be omitted, in which case it defaults to the
value returned by current-input-port. It is an error to read
from a closed port.
Previous: srfi shared structures rationale, Up: srfi shared structures [Index]